Various methods of slotted-ALOHA random access control on mobile units have been studied. For example, in a TDMA (time division multiple access) method, Idle-Single Casting Multiple Access with Partial Echo (ICMA-PE) in accordance with a digital car phone system standard (ARIB STD-27) or the like is being used. ICMA-PE itself is described in detail in Non-Patent Document 1.
Under random access control in TDMA such as PDC (Personal Digital Cellular) in Japan, a mobile station alternately performs transmission and reception according to the characteristics of TDMA. After data transmission by the mobile station, a base station can enable a condition as to whether or not transmission has been correctly performed by the mobile station to be reflected in reception subsequently performed by the base station, because a sufficient time period for doing so exists before the time at which reception is to be subsequently performed by the mobile station. Therefore no time slot loss occurs with respect to time. Also, the mobile station can perform transmission by full-duplex and therefore can be informed of the results of transmission to the base station by the next reception.
The base station may be configured so that if the size of data transmitted from the mobile station is so large that the time required for transmission extends over a plurality of slots, it determines the number of remaining slots from information contained in data in the leading slot, inhibits transmission by any other mobile station until the number of remaining slots becomes zero, and thereby makes a “reservation” to permit the mobile station that has transmitted the leading slot to perform transmission with priority.
In slotted-ALOHA random access control in an FDMA (frequency division multiple access) method, transmitted and received frames appear continuously with respect to time. A new concept is therefore required for notification (transmission), before a time for the next reception by a mobile station, of information as to whether or not a base station has correctly received data transmitted by the mobile station.
In systems under a strong demand for a reduction in price, e.g., wireless communication systems for business purposes, mobile stations are ordinary configured for half-duplex. A half-duplex-type mobile station cannot perform transmission during transmission and immediately after the completion of transmission due to switching operation between transmission and reception. Therefore, when a base station notifies such a mobile station about whether or not the base station has correctly received data in the leading frame that the mobile station has transmitted, it must transmit data indicating the reception result after a lapse of a certain time period from transmission of the leading frame by the mobile station.
Random access in FDMA is described in a narrow-band digital communication standard (ARIB STD-T61) for example. Also, a control method using the concept of reservation is described in Patent Document 1.
An example of operations in accordance with ARIB STD-T61 will first be described.
In ARIB STD-T61, a downstream frame from a base station to a mobile station contains information for collision control. This collision control information has the following contents:    1) I/B information indicating whether or not the next upstream time is available (with respect to the same frame number);    2) R/N information indicating whether or not an upstream signal having the third preceding frame number has been received; and    3) PE information indicating a partial echo of the upstream signal having the third preceding frame number.
In ARIB STD-T61, a mobile station can transmit three preceding frames without reception confirmation.
FIG. 11 shows an example of operations in a case where mobile stations MA and MB each transmit four consecutive frames at times generally coinciding with each other to cause collision therebetween. It is assumed that in this operation example the radio wave environment for the mobile station MA is better and information transmitted from the mobile station MB does not reach a base station. The operation in FIG. 11 will be described below.
When the mobile station MA receives the first downstream frame #1 from the base station, it determines that I/B (transmission permission/inhibition information) is I (permission) and starts transmission. An information length of 4 frames is recorded in data transmitted by the mobile station MA. On the other hand, when the mobile station MB receives the second downstream frame #2 from the base station, it determines that I/B therein is I and starts transmission. An information length of 4 frames is also recorded in transmitted data.
The base station receives the data transmitted from the mobile station MA and changes I/B to B in the fourth frame #4 since the transmitted data is consecutive data (having an information length larger than 1 and contained in a plurality of frames).
When the mobile station MA receives the fourth frame #4, it determines that R/N is R and that a CRC sent in the leading frame from itself and the received PE coincide with each other. It then determines that consecutive transmission can be continued, transmits the final frame #4, and stops transmitting since all the data has been transmitted.
On the other hand, the mobile station MB performs transmission without confirmation until it completes transmission of the third frame #3. However, the transmitted data does not reach the base station because of collision with transmission by the mobile station MA. When the mobile station MB receives a downstream frame (the fifth frame #5 in the example) after the completion of transmission of the third frame #3, it determines that the PE in the downstream frame does not coincide with the CRC sent in the leading frame from itself. At this point in time, the mobile station MB determines that the data transmitted from itself has not reached the base station, i.e., transmission failure, and tries to retransmit after random delay without transmitting the fourth frame.
A random access control method disclosed in Japanese Patent Application Laid-Open No. 2001-285928 will next be described with reference to FIG. 12.
In this random access control method, information for collision control is set together with other data in a downstream frame from a base station to a mobile station. The collision control information has the following contents:    1) I/B information designating permission/inhibition of transmission by a mobile station at an upstream time (with respect to the same frame number);    2) R/N information indicating whether or not an upstream signal having the third preceding frame number has been received; and    3) Mobile station information indicating from which mobile station the upstream signal having the third preceding frame number has been received.
According to an embodiment described in Japanese Patent Application Laid-Open No. 2001-285928, a mobile station is provided with I/B information in three-bit form. Details of this I/B information are as described below.    000: Transmission inhibition 1 (“Inhibition 1” in FIG. 8 of this publication) . . . There is only one remaining upstream transmission frame signal from a mobile station given a transmission right.    001: Transmission inhibition 2 (“Inhibition 2” in FIG. 8) . . . The final upstream transmission frame has been received or an error has occurred in reception of the upstream transmission frame signal.    010: Transmission inhibition 3 (“Inhibition 3” in FIG. 8) . . . An upstream transmission frame including a transmission request has been received from a mobile station.    100: Transmission right giving (“Giving” in FIG. 8) Giving a transmission right to a mobile station which has transmitted an upstream transmission frame signal of a transmission request without collision or error    101: Designated mobile station transmission permission (“Permission” in FIG. 8) . . . A state in which a particular mobile station is given a transmission right.    111: Available (“Available” in FIG. 8) . . . A state in which transmission of an upstream transmission frame signal from any mobile station is accepted.
FIG. 12 shows an example of transmission and reception of upstream and downstream frames in a case where mobile stations MA and MB each transmit four consecutive frames.
Operations for this transmission and reception will be described below by way of example with reference to FIG. 12.
When the mobile station MA receives a downstream frame (assumed to be first downstream frame #1) from a base station, it determines that I/B is “Available” and starts transmission. An information length of 4 frames is recorded in transmitted data.
The base station detects the signal by a time T1 at which the mobile station MA starts transmission, and makes I/B “Transmission inhibition 3” although reception of the leading frame is not completed. After receiving the entire leading transmission frame from the mobile station MA, the base station determines whether or not the data transmitted from the mobile station MA is consecutive data. In this example, since the information length is larger than 1 and since the transmitted data is consecutive data, the base station gives a transmission right to the mobile station MA at a time T2 and makes I/B “Transmission right giving” and makes the mobile station information designative of the mobile station MA. When the mobile station MA receives the third frame #3 including these items of information from the base station, it determines that transmission can be continued and transmits all the remaining frames. The mobile station MA thereafter stops transmitting since all the transmission frames have been transmitted.
On the other hand, at about the time for the second downstream frame #2, the mobile station MB has data to be transmitted. However, the mobile station MB performs a waiting operation since I/B in the downstream frame is “Transmission inhibition 3”. At a time T3 after the completion of transmission by the mobile station MA, the mobile station MB determines that I/B has become “Available” and starts transmission.
Non-Patent Document 1: Transactions of The Institute of Electronics, Information and Communication Engineers, vol. J76-B-II, No. 3, pp. 157-165 “Idle-signal casting multiple access with partial echo (ICMA-PE)”
Patent Document 1: Japanese Patent Application Laid-Open No. 2001-285928
The conventional slotted-ALOHA random access control is designed with a TDMA method in mind. However, it is difficult to perform the conventional slotted-ALOHA random access control as control for processing in a base station the contents of data transmitted from a mobile station and enabling reflection of a processing result in the next transmission from the mobile station because transmission and reception are continuously performed with respect to time in FDMA.
In a system in accordance with ARIB STD-T61, a mobile station can transmit three frames without reception confirmation by a base station. In this system, therefore, a mobile station can start transmission before another mobile station performing transmission completes transmission of leading three frame, so that collision occurs frequently between upstream frames.
In many cases of actual random access transmission from mobile stations, the number of frames is three or less. In transmission of three frames or less, I/B of the downstream frames from a base station is not changed from I. As a matter of fact, the chances of control by idle-signal casting are limited.
Random access control disclosed in patent document 1 has some effect in solving these problems. In random access control disclosed in patent document 1, no collision occurs between a particular mobile station and other mobile stations when the particular mobile station is continuously performing transmission with a transmission right given by a base station, but each and every mobile station is kept in a transmission inhibited state before given a transmission right. This control method therefore has a problem that the throughput is low. Also, this control method requires detecting a transmitted signal when a mobile station starts transmitting a leading frame, immediately providing collision control information reflecting a result of this detection, and immediately providing collision control information reflecting the completion of reception of one transmission frame by a base station after the completion of transmission of this transmission frame.
Thus, the load on a base station is large and there are difficulties in providing a base station configuration and control.
In the control method in accordance with ARIB STD-T61 and the control method disclosed in patent document 1, it is necessary for a mobile station to perform transmission/reception control in a full-duplex method. However, mobile stations such as wireless communication devices for business purposes under a strong demand for a reduction in price are ordinary configured for half-duplex. It is necessary to perform timing control in such half-duplex mobile stations by considering a transmission-reception switching time. Therefore the above-described control methods cannot be applied to such mobile stations.
Thus, the efficiency of the conventional random access control is low. There is a demand for random access control of higher efficiency.
The present invention has been achieved in consideration of the above-described problems of the conventional art, and an object of the present invention is to make possible random access control of high efficiency and high throughput.
Another object of the present invention is to make possible random access control with a reduced control load.
Still another object of the present invention is to make possible random access control applicable to a mobile station of a half-duplex configuration.